Transmission and reception of broadcast radio frequency signals within a structure, such as a building or a tunnel, is often a desirable feature in such apparatus as mobile communications gear and mobile medical monitors. However, a well known problem with using such apparatus within a structure is that the structure itself can interfere with proper reception by an intended receiver. Properties of a structure which cause this interference can include reflection, absorption and shielding of radio signals by the materials which compose the bulk of the structure. Equipment designers have therefore proposed apparatus for distributing reception or transmission equipment throughout a structure, so that the effects of these properties are lessened, using, for example, "leaky feeder", parallel feed and serial feed distributed antenna systems.
A "leaky feeder" system is a transmission system utilizing a coaxial feeder cable having strategically placed holes in the shielding of the cable, whereby some radio frequency energy injected into one end of the cable by a transmitter may "leak out", and thus be broadcast. A receiver may also be configured to use a "leaky feeder" antenna system. However, such a cable typically has large losses which can degrade signal/noise ratio by reducing signal amplitude in the presence of noise sources. Amplification can be used to restore acceptable signal levels, but signal/noise ratio remains poor, since noise at an amplifier input is boosted along with signal at the input. In fact, an amplifier typically injects additional noise into the network. Furthermore, this type of system typically has a signal/noise ratio which varies greatly with distance along the cable, producing variable performance in different parts of a given installation. High power levels used to obtain reasonable signal levels, the poor signal/noise ratio, and the signal/noise ratio variations make such a system costly and limit the usable length of the system.
Both serial feed and parallel feed distributed antenna networks share with the "leaky feeder" system the problem of losses in the feeder cables. In each of these approaches, a number of discrete antenna elements are placed at intervals, along, for example, a tunnel or building hallway. The elements are connected to a transmitter or receiver apparatus by either a feeder cable which connects each antenna to the next in a series connection, or parallel feeder cables, which each run the entire length from an antenna to the apparatus. Serial and parallel networks may be combined to form a tree topology. Parallel networks and tree topologies require many components in practical implementations of complete networks. This leads to high initial, installation and maintenance costs.
A further problem inherent in distributed antenna networks of the prior art is a lack of flexibility. For example, in an application in a hospital involving mobile medical monitors, changing facility use patterns may necessitate changes to the antenna network. For example, if patients wearing mobile monitors were previously allowed to walk around one area and that area is then relocated or extended to include a different hall or ward, the new hall or ward must be equipped with receiving antennas. Parallel networks and tree topologies would necessitate a different configuration, leading to increased cost and/or complexity. Increased complexity may lead to higher design, recalibration or installation effort to optimize performance. In particular, lack of flexibility substantially complicates the initial design of such antenna systems.
Therefore, it is an object of the present invention to provide a flexible distributed antenna system having a plurality of discrete antennas locatable, for example, within a structure such as a building which may be reconfigured easily, without necessitating recalibration, redesign, or extensive installation effort.
Another object of this invention is to provide a distributed antenna system having a high signal/noise ratio.
A further object of this invention is to provide such an antenna system which requires fewer components than prior art systems.
Yet another object of the present invention is to provide a distributed antenna system having feed network signal/noise ratio and gain essentially independent of which antenna within the system is considered.